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Silicon’s exceptional optical and electrical properties, combined with its high purity, crystal quality, abundance and low price makes it the main standard for developing new micro- and nanoscale technologies in electronics and photonics [1]. In this work, we present a novel application: the formation of vibrating microcavities by irradiating a Si (111)-oriented wafer covered with a micrometer-thick layer of SiO2, using single femtosecond laser pulses (120 fs, 800 nm) (Fig. 1(a)).
By using pulse fluences above the evaporation threshold of silicon, which causes a bulging of the SiO2 layer, microcavities of varying heights can be fabricated, giving rise to optical interference (Fig. 1(b)). To study the early stages of their formation, we employed time-resolved Femtosecond Microscopy. This pump–probe method enables the acquisition of reflectivity snapshots of the sample surface at different time delays of an illumination probe pulse (λprobe = 400 nm) after arrival of the pump, allowing the study of microcavity formation with sub-picosecond temporal resolution (Fig. 1(c)). The results reveal the presence of material melting and surface evaporation between layers at the Si/SiO2 interface, leading to a transient strong surface bulging, followed by subsequent cooling and contraction of the SiO2 layer to its final size.
Using other time-resolved optical probing techniques, such as Real-Time Reflectivity (RTR) measurements, the slower dynamics of microcavity formation can be studied, featuring complex long-lasting reflectivity oscillations (Fig. 1(d,e)). For delay times > 1 microsecond, periodic oscillations associated with membrane vibrations at well-defined resonance frequencies are observed. The microcavities have been re-excited using laser fluences below the modification threshold, demonstrating that the structures vibrate at specific frequencies in the MHz range (Fig. 1(f)). The oscillation frequency can be tuned smoothly by changing the laser fluence, demonstrating a high degree of control over the cavity dynamics, and results have been compared with theoretical predictions, yielding good agreement. These unique structures might find applications as acoustic resonators.